The present invention relates to a method and an apparatus for exchanging humidity between the fuels in a fuel cell. A fuel cell has a stack-like structure with an electrolyte arranged between end plates. Between the electrolyte and one end plate is an anode, and between the electrolyte and the other end plate is a cathode. Solid and liquid electrolytes are known; as the case may be, the electrolyte can be absorbed by a carrier structure or itself possesses the requisite solidity enabling it to be used in the cell. The operating temperatures also differ considerably and vary from ambient temperature to several hundred degrees C. and above.
It is customary to combine individual fuel cells to form a fuel cell stack, in order to obtain the desired operating voltage by connecting a suitable number of individual cells in series.
In one of the many known structural forms to which the invention can be applied, a polymer electrolyte membrane (PEM) is used in the fuel cell. In operation the fuel cell is then supplied on the anode side with e.g. hydrogen in gaseous form and on the cathode side with e.g. ambient air containing oxygen. The hydrogen and oxygen are used here by way of a non-limiting example.
In the present case for the sake of simplicity all reaction partners which participate in the chemical reaction in a fuel cell are designated as fuels, as are their carrier fluids (e.g. ambient air as a carrier fluid for the reaction partner O2). The fuels are present in fluid form.
In the presence of a catalytic converter, e.g. platinum, the hydrogen molecules split into protons and electrons, while on the cathode side oxygen molecules absorb electrons and are ionized into O2− ions. Since the membrane is designed as a proton conductor, the protons formed on the anode side diffuse through the membrane to the cathode; the electrons are supplied to the cathode via a separate electrical conductor, with an electrical load being inserted. The protons and oxygen ions react there to become water.
The protons are transported in the membrane in the form of H3O+ ions; this is dependent upon the appropriate water content of the membrane. With a higher water content the ability of the membrane to conduct protons improves. The degree of operationally necessary humidity is therefore predetermined by the person skilled in the art, taking into account the parameters of the fuel cell and characteristics of the membrane.
If the humidity is not substantially constant across the membrane, the fuel cell cannot be kept at an optimum operating point; areas of the membrane which are too dry reduce the production of current, since dryness makes it more difficult for protons to be transported through the membrane.
In operation, the membrane continuously loses humidity, since as stated above, the protons are transported through the membrane via H3O+ ions. Furthermore, water is produced on the cathode side. In this way the air flow on the cathode side continuously becomes charged with humidity on its route across the membrane. In the area of the inlet, the air flow which is not as yet charged with humidity helps the membrane to dry out, while in the area of the outlet it tends to have excess humidity (condensation).
As a result, the membrane is too dry on the inlet side, which as mentioned above is undesirable. If a fuel cell stack continues to be operated, the overly dry parts of the membrane of a fuel cell generate resistance via a corresponding production of heat, which, in turn, accelerates the drying out process. This leads to a significant reduction in cell output, overheating, and even destruction of the affected cell.
Humidity refers not only to the presence of water. Just as the term “fluids” is used here to refer to all possible fuels, “humidity” refers to any substance which in the case of a given membrane (and given fuels) permits charges (such as protons in the case of PEM membranes) to be transported through the membrane.
U.S. Pat. No. 4,973,530 discloses a method for regulating the humidity content in a fuel cell membrane by continuous addition of externally supplied water into the anode area and by continuous removal of humidity on the cathode side. For the addition of water a further membrane which is pervious to water is arranged next to the membrane of the fuel cell and the channel for the hydrogen is arranged in such an alternating pattern that it is routed back and forth in a zigzag fashion first via the membrane of the fuel cell, then via the water-pervious membrane, until both membranes are fully covered and the residual hydrogen can be expelled to the outside. A further, also water-pervious membrane is used to remove humidity from the cathode area with the same zigzag routing arrangement, in that a dry gas is routed over the further membrane, absorbs the humidity to be discharged, and expels it to the outside.
It is further proposed to use hydrogen as a dry gas which is to be supplied to the anode area, with the advantage that the hydrogen then already contains a certain humidity before entering the area of the membrane of the fuel cell. To construct a stack of fuel cells it is proposed to create common supply and discharge channels routed through the stack for all anode-side and cathode-side areas, including the areas conveying water (for humidification) or a dry gas (for dehumidification), in that the same fluid-conveying lines are connected together through correspondingly cut holes in the membranes.
It is now the object of the present invention to provide an improved fuel cell or a stack of fuel cells with controlled humidity management.